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Abstract:

The present invention provides an abrasive treatment technique capable of
planarizing at an extremely high rate silicon carbide, which is thermally
and chemically extremely stable and for which it is extremely difficult
to efficiently perform an abrasive treatment. The present invention is a
polishing slurry for silicon carbide wherein the polishing slurry
includes a suspension liquid in which the pH thereof is 6.5 or more and
manganese dioxide particles are suspended. The polishing slurry for
silicon carbide is preferably a suspension in which manganese dioxide
particles are suspended in an aqueous solution allowed to have a redox
potential falling in a range enabling manganese to be present as
manganese dioxide. The redox potential V of the polishing slurry
preferably falls within the range specified by the following formula
representing a relation between V and pH, pH being a variable:
1.014-0.591 pH≦V≦1.620-0.0743 pH

Claims:

1. A polishing slurry for silicon carbide wherein the polishing slurry
comprises a suspension liquid in which the pH thereof is 6.5 or more and
manganese dioxide particles are suspended.

2. The polishing slurry for silicon carbide according to claim 1, wherein
manganese dioxide particles are suspended in an aqueous solution allowed
to have a redox potential falling in a range enabling manganese to be
present as manganese dioxide.

3. The polishing slurry for silicon carbide according to claim 1, wherein
the redox potential V falls within the range specified by the following
formula representing a relation between V and pH, pH being a variable:
014-0.591 pH≦V≦1.620-0.0743 pH

5. The polishing slurry for silicon carbide according to claim 2, wherein
the redox potential V falls within the range specified by the following
formula representing a relation between V and pH, pH being a variable:
1.014-0.591 pH≦V≦1.620-0.0743 pH

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Filed of the Invention

[0002] The present invention relates to a technique for polishing silicon
carbide, in particular, a polishing slurry for silicon carbide and a
polishing method for polishing silicon carbide using as a polishing
material manganese dioxide particles.

[0003] Description of the Related Art

[0004] Recently, silicon carbide (SiC) and silicon nitride
(Si3N4) have been attracting attention as substrate materials
for power electronics semiconductors and white LEDs. These substrate
materials are extremely high in hardness, and are known as
difficult-to-polish materials. Accordingly, when a substrate for use in
epitaxial growth is produced, in general, the surface of the substrate is
planarized by performing an abrasive treatment for a long period of time,
for the purpose of realizing a high degree of surface precision.

[0005] In such an abrasive treatment, for example, silica (see, for
example, Patent Document 1) used for polishing silicon semiconductor
substrates and diamond fine particles (see, for example, Patent Document
2) higher in hardness than silicon carbide and silicon nitride are used
as polishing materials. There have also been proposed polishing methods
in which chemical reactions are utilized by irradiating titanium oxide
and cerium oxide with light. However, as matters now stand, such
conventional polishing treatments take dozens of hours for polishing
silicon carbide or silicon nitride because the material hardness of
silicon carbide or silicon nitride as an object to be polished is too
high.

[0006] As a technique for polishing silicon carbide substrates, the use of
manganese dioxide (MnO2) as a polishing material has also been known
(see, for example, Patent Document 3 and Patent Document 4). In such
prior art techniques, for the purpose of taking advantage of dimanganese
trioxide, regarded as higher in an abrasive capability than manganese
dioxide, dimanganese trioxide is formed on the surface of manganese
dioxide particles by calcination or the like of manganese dioxide, and
silicon carbide is polished with the thus treated manganese dioxide.
According to the abrasive treatment utilizing dimanganese trioxide as in
such prior art techniques, silicon carbide can be polished at a
comparatively high rate; however, there has been strongly demanded a
polishing technique enabling an abrasive treatment at a further higher
rate.

[0011] The present invention has been achieved against a background of
such circumstances as described above, and an object of the present
invention is to provide an abrasive treatment technique capable of
planarizing at an extremely high rate silicon carbide, which has a
hardness next to that of diamond and is thermally and chemically
extremely stable and for which it is extremely difficult to efficiently
perform an abrasive treatment.

Means for Solving the Problems

[0012] The present inventors made a diligent study on the utilization
technique of manganese oxide used in an abrasive treatment of silicon
carbide. Consequently, the present inventors have thought up the present
invention by discovering that polishing of silicon carbide with manganese
dioxide particles in manganese oxide dramatically improves the polishing
efficiency.

[0013] The present invention relates to a polishing slurry for silicon
carbide wherein the polishing slurry includes a suspension liquid in
which the pH thereof is 6.5 or more and manganese dioxide particles are
suspended.

[0014] In the polishing slurry for silicon carbide according to the
present invention, manganese dioxide particles are preferably suspended
in an aqueous solution regulated to have a redox potential range allowing
manganese to be present as manganese dioxide.

[0015] As is known from the phase equilibrium diagram (potential-pH
diagram, see FIG. 1) (Atlas of Electrochemical Equilibria in Aqueous
Solutions, Marcel Pourbaix, Pergamon Press Cebelcor(1996)) showing the
redox reaction of manganese, manganese (Mn) becomes manganese dioxide
(MnO2), dimanganese trioxide (Mn2O3), trimanganese
tetraoxide (Mn3O4) or permanganate ion (MnO),4- depending
on the pH and the redox potential at the pH. In conventional polishing
methods using manganese dioxide, an abrasive treatment has been
performed, not under the conditions that manganese dioxide alone is
present, but under the conditions that dimanganese trioxide
(Mn2O3) and permanganate ion (MnO4.sup.-) are mixed with
manganese dioxide due to calcination, chemical reaction or the like. In
particular, in Patent Document 3, silicon carbide is polished under the
conditions that the redox potential and the pH are positively regulated
so as to allow the formation of dimanganese trioxide (Mn2O3).
In contrast to this, according to the study of the present inventors, it
has been revealed that the efficiency of polishing of silicon carbide is
dramatically improved by polishing silicon carbide with a polishing
slurry in which intentionally the condition allowing the presence of
manganese dioxide is maintained.

[0016] The range allowing manganese dioxide to be stably present can be
specified on the basis of the phase equilibrium diagram (potential-pH
diagram) showing the redox reaction of manganese (Mn). In the present
invention, the redox potential value V allowing manganese dioxide to be
stably present in a pH range of 6.5 or more preferably satisfies the
following formula representing a relation between the redox potential V
and pH:

1.014-0.0591 pH≦V≦1.620-0.0743 pH

[0017] In the present invention, the pH within the range in which
manganese dioxide can be stably present is specified to be 6.5 or more.
According to the study performed by the present inventors, it has been
found out that when silicon carbide is polished with manganese dioxide
particles, the higher the pH of the polishing slurry is, the more the
abrasive capability of the polishing slurry is improved. In the present
invention, the pH is preferably 6.5 or more, more preferably 8 or more
and furthermore preferably 10 or more.

[0018] The manganese dioxide in the present invention is not particularly
limited with respect to the production method thereof. Manganese dioxide
obtained, for example, by the following production methods can be used: a
so-called electrolytic method in which manganese dioxide is produced by
electrolyzing an electrolyte solution containing manganese (Mn) ion to
form oxide on an anode; and a chemical method in which a water-soluble
manganese (Mn) salt is neutralized and precipitated with a carbonate or
the like, and the precipitate is oxidized. The electrolytic method can be
said to be a preferable method because the manganese dioxide produced by
the anodic deposition method based on electrolysis is favorably strong
with respect to the deposited particles thereof. In the manganese dioxide
in the present invention, the average particle size thereof is preferably
0.1 to 1 μm. This is because when the average particle size exceeds 1
μm, the polishing material tends to be precipitated to cause the
occurrence of the concentration unevenness in the polishing material
slurry and the occurrence of polishing failure. The average particle size
as referred to herein is determined by a laser diffraction/scattering
particle size distribution measurement method, and the average particle
size is the particle size D50 of the volume-based cumulative
fraction of 50% in laser diffraction/scattering particle size
distribution measurement.

[0019] The polishing slurry for silicon carbide according to the present
invention can be realized by suspending in water manganese dioxide having
a predetermined average particle size. The suspension liquid in which
manganese dioxide is suspended in water (a so-called polishing material
slurry or a so-called polishing slurry) can be subjected to a wet
pulverization treatment, if necessary. The manganese dioxide
concentration in the suspension liquid is preferably set at 1 wt % to 20
wt %. The pH of the suspension liquid can be regulated by adding
potassium hydroxide, ammonia, hydrochloric acid or the like. The redox
potential can be regulated with hydrogen peroxide water or the like.

Effect of the Invention

[0020] As described above, according to the present invention, an abrasive
treatment is enabled in which silicon carbide, for which it has been
assumed to be extremely difficult to perform an abrasive treatment, is
planarized at an extremely high rate.

[0022] FIG. 2 is a microgram of an AFM measurement of the polished surface
in Example 1.

[0023] FIG. 3 is a microgram of an AFM measurement of the polished surface
in Comparative Example 3.

MODES FOR CARRYING OUT THE INVENTION

[0024] The best mode for carrying out the present invention is described
with reference to Examples and Comparative Examples.

EXAMPLE 1

[0025] In Example 1, manganese dioxide was deposited on an anode by
electrolysis of an aqueous solution of manganese sulfate. The deposit was
collected and disintegrated with a pin mill (Atomizer, manufactured by
Powrex Corp.), and then subjected to a dry pulverization treatment with a
jet mill (PJM-200SP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) to
yield a manganese dioxide powder having an average particle size of 0.5
μm. The average particle size was measured with a laser
diffraction/scattering particle size distribution analyzer (LA-920,
manufactured by Horiba, Ltd.). A manganese dioxide slurry was prepared by
dispersing the manganese dioxide powder in purified water so as for the
slurry concentration to be 10 wt %. In the manganese dioxide slurry of
Example 1, the pH was 6.7 and the redox potential was 0.832 V.

[0026] By using the manganese dioxide slurry of Example 1, the following
polishing test was performed. As the polishing object, a single crystal
substrate of SiC (silicon carbide) of 2 inches in diameter was used. The
polishing method was as follows: a non-woven fabric pad (SUBA-400,
manufactured by Nitta Haas Inc.) was bonded to a single surface polisher
for CMP (platen diameter: 25 cm, number of rotations: 90 rpm), a load of
18.3 kPa (187gf/cm2) was applied, and polishing was performed for 12
hours. During the 12-hour polishing, polishing was performed while the
polishing material slurry was being circulated. The stock removal was
calculated by measuring the substrate weights before and after polishing.
When the stock removal of Comparative Example 1 shown below was
represented by 10 as the reference, the stock removal of Example 1 was
five times the stock removal of Comparative Example 1, namely, 50 (See
Table 1).

EXAMPLE 2

[0027] A manganese dioxide slurry having a pH of 9.0 and a redox potential
of 0.733 V was prepared as follows: the same manganese dioxide powder as
in Example 1 was used; the manganese dioxide powder was dispersed in
purified water so as for the slurry concentration to be 10 wt %; and
potassium hydroxide was added to the slurry to regulate the pH of the
slurry. A polishing test was performed under the same conditions as in
Example 1, and the stock removal was examined. The result thus obtained
is shown in Table 1.

EXAMPLE 3

[0028] A manganese dioxide slurry having a pH of 10.1 and a redox
potential of 0.688 V was prepared as follows: the same manganese dioxide
powder as in Example 1 was used; the manganese dioxide powder was
dispersed in purified water so as for the slurry concentration to be 10
wt %: and potassium hydroxide was added to the slurry to regulate the pH
of the slurry. A polishing test was performed under the same conditions
as in Example 1, and the stock removal was examined. The result thus
obtained is shown in Table 1.

EXAMPLE 4

[0029] A manganese dioxide slurry having a pH of 11.9 and a redox
potential of 0.561 V was prepared as follows: the same manganese dioxide
powder as in Example 1 was used; the manganese dioxide powder was
dispersed in purified water so as for the slurry concentration to be 10
wt %: and potassium hydroxide was added to the slurry to regulate the pH
of the slurry. A polishing test was performed under the same conditions
as in Example 1, and the stock removal was examined. The result thus
obtained is shown in Table 1.

COMPARATIVE EXAMPLE 1

[0030] For comparison, the same polishing test as in Example 1 was
performed by using a commercially available colloidal silica (Compol 80,
average particle size: 70 nm to 80 nm, manufactured by Fujimi Inc.). A
colloidal silica slurry was prepared by dispersing the colloidal silica
in purified water so as for the slurry concentration to be 10 wt %. A
polishing test was performed under the same conditions as in Example 1,
and the stock removal was examined. The stock removal of Comparative
Example 1 was defined as 10 (unitless), on the basis of which the stock
removal values of Examples and Comparative Examples were presented as
numerical values. The results thus obtained are shown in Table 1.

COMPARATIVE EXAMPLE 2

[0031] In Comparative Example 2, a polishing test was performed by using
the manganese dioxide powder (average particle size: 0.5 μm) of
Example 1 calcined at 850° C. (1 hour). X-ray diffraction
identification of the crystal structure of the manganese dioxide having
been calcined verified that the calcination resulted in dimanganese
trioxide (Mn2O3). After calcination, with a bead mill, the
calcined product was subjected to a pulverization treatment until the
average particle size reached 0.4 μm. A dimanganese trioxide slurry
was prepared by dispersing the thus pulverized dimanganese trioxide
powder in purified water so as for the slurry concentration to be 10 wt
%. In the dimanganese trioxide slurry of Comparative Example 2, the pH
was 5.9 and the redox potential was 0.604 V. By using this slurry, a
polishing test was performed under the same conditions as in Example 1,
and the polishing rate was examined. The result thus obtained is shown in
Table 1.

COMPARATIVE EXAMPLE 3

[0032] In Comparative Example 3, a manganese dioxide powder (average
particle size: 0.5 μm) obtained in the same manner as in Example 1 was
dispersed in water to prepare a slurry. Hydrochloric acid was added to
the slurry to regulate the pH of the slurry to be 4.5. The redox
potential of the manganese dioxide slurry of Comparative Example 3 was
0.900 V. According to the phase equilibrium diagram (potential-pH
diagram) representing the redox reaction of manganese (Mn), shown in FIG.
1, the redox potential and the pH indicate that the particles in the
slurry are manganese dioxide particles. The redox potential and the pH of
Comparative Example 3 satisfy the formula 1.014-0.0591
pH≦V≦1.620-0.0743 pH representing a relation between V and
pH; however, the concerned pH value is a value lower than pH 6.5. The
result of the polishing test in Comparative Example 3 is shown in Table
1.

[0033] As shown in Table 1, it has been revealed that in each of Examples,
the stock removal for the same period of time is extremely larger as
compared to the cases where conventional colloidal silica and the like
were used, and the polishing rate is extremely high. It has also been
revealed that the larger the pH value of the slurry is, the more the
stock removal (polishing rate) is improved. As shown in Comparative
Example 3, it has been verified that when the pH falls out of the range
of the present invention, the polishing efficiency tends to be degraded.

[0034] FIG. 2 shows the measurement results of the surface roughness of
the polished surface of the substrate after the polishing test in Example
1 with an AFM (atomic force microscope, Nanoscope IIIa, manufactured by
Veeco Instruments, Inc.). In Example 1, the polished surface was finished
extremely flat and smooth, and the surface roughness Ra thereof was 0.281
nm. In contrast to this, it has been verified that the condition of the
polished surface in Comparative Example 3 was considerably rough as
compared to that in Example 1.

INDUSTRIAL APPLICABILITY

[0035] The present invention enables extremely efficient polishing
processing of silicon carbide (SiC) used as a substrate material for
power electronics semiconductors and white LEDs.